Lionel only has the power supply with the barrel connector on the 6-81603 (4 amp wall pack). One would need to measure the barrel connector and then get one to make your own.
My LionChief Thomas set box is buried. I have never used the DC wall pack. If you get stuck, I'll dig it out and measure the connector.
Is the bottom of the block a screwed on panel? Perhaps you could get to the terminals and connect there.
email forum member Mikado and ask about a size or option.